(1) Field of the Invention
The present invention relates to a distance measurement system for collision avoidance of aircraft. The system is to prevent aircraft collision by suitably providing, for instance, a threat signal depending on a result of measurement of mutual distance between proximate or approaching aircraft in flight should danger of collision occur.
(2) Description of the Prior Art
According to recent development of air traffic, the danger of collision between aircraft has been increased very much because more and more large numbers of various aircraft take the air of same area and same altitude within the same time zone. Therefore it is a very important to establish and maintain safe navigation of aircraft. There are provided ground radar units mainly for the air traffic control purposes. But it would cost an enormous amount to establish a safe navigation control system using only such ground radar. In addition a number of technical difficulties must, yet be solved in realizing such a system. Furthermore, such a control system is not effective outside the range of the ground radar.
The conventional distance measurement system including that using a secondary surveillance radar of a subject aircraft may be classified into two major systems, i.e., a passive system and an active system.
A passive distance measurement system is a system based on a principle of measurement of incoming information only. In this system, a radar beam from a ground station functioning as a secondary surveillance radar and air traffic control (ATC) transponder outputs of the subject aircraft and of other nearly, i.e. threatening aircraft delivered in response to the ground secondary surveillance radar are utilized for identifying the distance between the two aircraft.
In more detail this system is mainly based on the measurement of a time difference between an ATC transponder output of the subject aircraft delivered in response to a ground secondary surveillance radar beam of a ground station and an ATC transponder output of the other nearby aircraft delivered in response to the same ground radar beam.
The passive system is further based on the following facts.
(1) The locus of the points at which the above time difference becomes constant takes the form of ellipsoid having the subject transponder antenna and the ground antenna as the focuses. PA1 (2) ATC transponders of the subject and the other aircraft can only receive the ground radar beam when the two aircraft are covered by an effective radiation pattern from the ground radar beam. PA1 (3) The transmitting wave strength of an ATC transponder is a certain value decided by regulation so that the reception wave strength of the ATC transponder output of other aircraft can be used to represent a function of distance between the two aircraft. PA1 (1) Fruit noise of an overall radar system may be increased by the interrogation of the secondary surveillance radar which is mounted on the aircraft and by the response signal thereto. PA1 (2) During the interrogation and response between the subject aircraft and the other or threatening aircraft aircraft, the ATC transponder on the second aircraft becomes insensitive for the further interrogation and therefore the function of the secondary surveillance radar is spoiled. PA1 (a) decrease the number of interrogation signals from the subject aircraft. PA1 (b) suppress radiation power of the interrogation signal.
This passive distance measurement system has certain disadvantages.
In the system of deducing the distance by obtaining the above time difference, there will be no time difference when the second aircraft is located on a line connecting the subject aircraft and the ground antenna or comes very close to said line. In this case the time difference becomes same as the time required for the response to the transmission of the ATC transponder from the second aircraft irrespective of the distance between the second aircraft to the subject aircraft.
To assume the distance between aircraft, by the level of received wave strength set forth above (3) also has a certain danger. Because the electric field strength of the ATC transponder of the second aircraft may not have a constant value and it may vary depending on the course and attitude of flight of the second aircraft mainly by a reason of non-uniformity of the transmission pattern of the antenna of the ATC transponder.
An active distance measurement system overcomes most of the abovementioned disadvantages of the passive system. In an active distance measurement system, the ground station and its transmission radar beam pattern are not utilized, but the collision avoidance system on the subject aircraft is given the facility of transmitting an interrogation signal acting as a kind of secondary surveillance radar. By measuring the time difference between the transmission of the interrogation signal and reception of a response signal delivered from the ATC transponder of the second aircraft responding thereto by the collision avoidance system of the subject aircraft, the distance between the two aircraft can be obtained. This active distance measurement system affords a substantially high accuracy in the distance measurement compared with the passive distance measurement system.
It is preferred to arrange the transmission of the secondary surveillance radar, in the active distance measurement system equipped on an aircraft, during an interval of scanning of the ground radar beam, which has a very sharp directivity. By the above arrangement, the influence of the transmitted signals of the subject and other aircraft to the ground station can be minimized. The applicants had disclosed abovementioned arrangement in Japanese Patent Application Publication No. 29,358/73. There is also a possibility that both the subject and the threatening aircraft are navigating in a zone where no ground radar exists. In such a case, the secondary surveillance radar on the aircraft may be arranged to transmit the interrogation signal at a certain time interval irrespective of the existence of other approaching aircraft.
In the active distance measurement system, since a highly accurate distance measurement is possible, the system may be modified to have a function of measuring the variation of the abovementioned time difference. In this case mutual speed of the two aircrafts can be obtained without much difficulty and this would contribute in avoiding collision.
Thus the active distance measurement is a much improved system compared with the passive distance measurement system, however, this system has still disadvantages substantially mentioned below.
In order to avoid aforementioned disadvantages of the active distance measurement system either of the following two steps must be taken;